One sure way to tell that “The Sessions” is bound to be an exceptional movie is that its subject sounds like nothing anyone would want to see.

The story of a man in an iron lung who decides that he wants to experience sex could not have gotten funding on its premise alone. Obviously, there had to be something there, and there is.

“The Sessions” is moving. At times, it's even erotic, which is unexpected, to say the least. It sends viewers out of the theater with a heightened sense of the physical and a real feeling for all the things that sex means in human life.

“The Sessions” goes beyond what movies usually deal with when they talk about love — attraction, the mating dance, the happily-ever-after. It's about people's most basic need to connect, express and feel through sexuality. The film is raw and adult and, in the least somber way imaginable, unusually dignified.

Based on the real-life story of Mark O'Brien, a Bay Area poet and journalist who died in 1999, it dramatizes a period in the 1980s when, at 38, O'Brien hired a sex therapist. Among the ancillary revelations of “The Sessions,” for those of us previously in ignorance, is that hands-on sex therapy is nothing like prostitution.

Part therapist, part sex partner, part life coach, the therapist (Helen Hunt) ministers to a severely disabled patient and makes him feel as if he's part of the world. She helps bring him into the world. If anything, this is like missionary work.

John Hawkes plays the entire role on his back. (You might find yourself turning your head sideways to get a look at his face.) As played by Hawkes, O'Brien is a sweet and witty person who wants to get as much out of life as he can.

A polio victim, he can only move his head and needs the iron lung to breathe, though he can go as long as three hours outside of it. He is about as physically limited as a person can be, but with all the longings, even the romantic longings, of an able-bodied person.

Written and directed by Ben Lewin, “The Sessions” is most impressive in the way its tone harmonizes a whole range of different scenes and moods. The scenes between the devout O'Brien and his parish priest (William H. Macy), in which he discusses his plans to pursue sex therapy, are funny in a straightforward comic way. But the sex-therapy scenes — the sessions that give the film its title — are clinical, explicit and wrenching.

People will leave the theater talking about Hawkes, about the role's physical difficulty, Hawkes' emotional nakedness, and the way he conveys, just in his demeanor, a history of lifelong disability. But this is a movie with two remarkable performances, and in “The Sessions” Hunt gets her best screen opportunity in years.

As you might expect, Hunt plays the sex therapist with her usual matter-of-fact directness, but she adds something else, something skewed — the eccentricity that could allow her to do this kind of work.

It's also worth noting that in those sexual sessions, Hunt is the one who is completely naked, under brutal lighting that would give a 25-year-old pause. She looks great, but make no mistake: Hawkes may be depicting bravery in “The Sessions,” but Hunt is the one who's being brave.